1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a method for providing 4 quasi-orthogonal functions (QOF), which are used in association with 256 Walsh codes in a forward link, in a base station transceiver system (BTS) and a base station controller (BSC).
2. Description of the Related Art
Existing IS-95A/B CDMA communication systems spread a radio channel with a Walsh function. However, as the IS-2000 standard introduces new channels to the forward and reverse links, the existing conventional communication system lacks the necessary amount of Walsh codes to maintain orthogonality between channels
Accordingly, there is a need for a method for increasing the number of the channels, without decreasing the orthogonality between the existing channels. For the IS-2000 forward link, several methods have been proposed. One method is to define a quasi-orthogonal function (QOF) necessary for the mobile station thereby to assign 4 times the channels as compared with the case when the existing Walsh function is used, and another method is to expand the existing Walsh function which can generate 64 Walsh codes, so as to generate 256 Walsh codes. The quasi-orthogonal function (QOF) is commonly applied to IS-2000 forward channels, such as the fundamental channel (FCH), the dedicated control channel (DCCH) and the supplemental channel (SCH). However, existing base stations can only support 64 Walsh code channels.
Now, reference will be made to the types of the channels used in the IMT-2000 (International Mobile Telecommunications 2000) standard. IMT-2000 includes UMTS (Universal Mobile Telecommunication Service) and CDMA-2000/IS-2000.
Every channel is divided into a physical channel and a logical channel. The logical channel is established over the physical channel, and several logical channels can be established on a single physical channel. If the physical channel is released, the logical channel established over the physical channel is automatically released. It is not necessary to establish another physical channel in order to establish a certain logical channel. When a physical channel to be established for a logical channel is already established for another logical channel, the only required operation is to assign this logical channel to the previously established physical channel.
The physical channel can be divided into dedicated channels and common channels according to its characteristics. Dedicated channels are exclusively used for communication between the BTS and a particular mobile station (MS), and include a fundamental channel (FCH), a dedicated control channel (DCCH) and a supplemental channel (SCH). The fundamental channel is used to transmit voice, data and signaling signals. Such a fundamental channel is compatible with TIA/EIA-95-B. The dedicated control channel is used to transmit data and signaling signals. The dedicated control channel supports a discontinuous transmission (DTX) mode in which data is only transmitted when the upper layer generates transmission data. Because of this property, the dedicated control channel is suitable for effectively providing a packet service. The supplemental channel is used to transmit large amounts of data.
In addition to the dedicated channels stated above, the physical channel includes a common channel which is used in common by the base station and several mobile stations. A physical channel for the forward link transmitted from the BTS to the MS is called a paging channel, and a physical channel for the reverse link transmitted form the MS to the BTS is called an access channel. These common channels are compatible with IS-95B.
The logical channels to be assigned on the above physical channels include a dedicated signaling channel (dsch) and a dedicated traffic channel (dtch). The dedicated signaling channel can be assigned to the fundamental channel and the dedicated control channel, which are physical channels. The dedicated traffic channel can be assigned to the fundamental channel, the dedicated control channel and the supplemental channel. The dedicated signaling channel is used when the base station and the mobile station exchange a control signal. The dedicated traffic channel is used when the base station and the mobile station exchange user data.
The common logical channel to be assigned on the common physical channel is divided into a common signaling channel (csch) used to transmit control signal, and a common traffic channel (ctch) use to transmit user data. The common logical channels are assigned on the paging channel for the forward link, and are assigned on the access channel for the reverse link.
FIG. 1 shows a structure of a general mobile communication system. More specifically, FIG. 1 shows a reference model of a 3G IOS (Interoperability Specifications) for a digital air interface between a mobile switching center (MSC) and a base station, and between base stations in the common mobile communication system.
Referring to FIG. 1, between MSC 20 and BSC 32, a signal is defined as an A1 interface and user information is defined as an A2/A5 (circuit data) interface. An A3 interface is defined to connect a target BS 40 to a frame selection/distribution unit (SDU) function block 34 of a source BS 30 for soft/softer handoff between base stations. The signaling and user traffic between the target BS 40 and the SDU function block 34 of the source BS 30 are transmitted through the A3 interface. An A7 interface is defined for signal exchange between the target BS 40 and the source BS 30, for soft/softer handoff between the base stations. In the CDMA mobile communication system, a wired communication link between the base station 30 and the base station 40, and between the base station 30 and the MSC 20, is comprised of a forward link transmitted from the MSC 20 to the base station 30, a reverse link transmitted from the base station 30 to the MSC 20. Generally, a wired bi-directional line connected between the MSC 20 and the base station 30 carries the forward and reverse links. The MSC 20 includes a call control and mobility management block 22 and a switching block 24. Further, the MSC 20 is connected to a data network such as the Internet through an interworking function (IWF) block 50. The wired line exists over all the interfaces in the RAN, e.g., MSC-BSC, BS-BS, BSC-BTS, BSC-Target BTS and so on.
FIG. 2 shows a procedure for exchanging signals between the BTS and the BSC (more specifically, the SDU function block in the BSC, BSC-SDU) according to the prior art. The operation can be performed either between the BSC 32 (or BSC-SDU 34) and the BTS 36 in the source BS 30, or between the BSC 42 and the BTS 44 in the target BS 40. FIG.2 is independent of types of the required physical channel, i.e., DCCH, FCH, SCH or whatever. That is, the invention in this document can be applied to all types of physical channels.
Referring to FIG. 2, the BTS determines forward and reverse channels to be established with the MS and then generates a signaling message (more specifically, a connect message) necessary for channel establishment, in step 201. The generated signaling message includes a frame selector (or channel type) and channel information. The detailed operation of step 201 will be described later with reference to FIG. 3. The BTS sends the generated connect message to the BSC in step 203. Upon receipt of the connect message, the BSC analyzes the received connect message to check the channel assigned to the MS, and generates a connect ACK message to be transmitted to the BTS, in step 205. The detailed operation of receiving the connect message will be described later with reference to FIG. 4. The BSC sends the generated connect ACK message to the BTS in step 207. The connect ACK message includes information for acknowledging establishment of the channel requested by the BTS. The BTS then assigns the acknowledged channel to the MS in step 209.
Summarizing the operation of FIG. 2, the BTS generates the connect message including information about a channel to be assigned to the MS and sends the generated connect message to the BSC. Upon receipt of the connect message, the BSC processes the received connect message, generates the connect ACK message for acknowledging channel assignment and sends the generated connect ACK message to the BTS. The BTS then assigns the acknowledged channel to the MS.
FIG. 3 shows a detailed procedure for transmitting the connect message according to the prior art. This operation is performed when the BTS transmits the connect message to the BSC-SDU when it is required to assign a channel to the MS. The structures of the connect message transmitted from the BTS to the BSC-SDU are shown in FIGS. 5A and 5B.
Referring to FIG. 3, the BTS determines in step 301 whether the channel to be assigned to the MS is a supplemental code channel (SCCH). SCCH is the IS-2000 defined name corresponding to IS-95B SCH (Supplemental Channel). IS-2000 is on the evolution path of IS-95A/B. IS-2000 also has the IS-95 A/B channels in order to maintain backward compatibility. It is determined in step 301 that the channel to be assigned to the MS is SCCH, the BTS sets (designates) in step 303 the frame selector (or channel type) in the connect message, whose structure is shown in FIGS. 5A and 5B, to IS-95B SCCH, so as to enable the BSC to recognize that the channel to be assigned is an IS-95B channel, and then designates a Walsh code with 6-bit channel information. Thereafter, in step 305, the BTS ignores an information element overlapped due to establishment of the IS-95A/B fundamental channel in the connect message, whose structure is shown in FIGS. 5A and 5B. Cell Information, Extended Handoff Parameters in the A3 Connect Information element are overlapped with those in the same Connect message used when IS-95 A/B FCH is newly established. IS-95B SCCH establishment procedure follows the IS-95 A/B FCH establishment procedure. IS-95B SCCH has to be established in parallel to IS-95A/B FCH under the same cell. The BTS fills all other information elements to complete the connect message, and then transmits the connect message to the BSC. Here, in a handoff (HO) situation, the BTS fills all the handoff-related information element.
If it is determined in step 301 that the channel to be assigned to the MS is not SCCH, the BTS designates in step 307 the frame selector (or channel type) in the connect message shown in FIGS. 5A and 5B to the fundamental channel, so as to enable the BSC to recognize that the channel to be assigned is an IS-95B fundamental channel, and then designates a Walsh code with 6-bit channel information. Thereafter, in step 309, the BTS fills all the information elements in the connect message of FIGS. 5A and 5B to complete the connect message, and then transmits the connect message to the BSC. Here, in the handoff (HO) situation, the BTS fills all the handoff-related information element.
FIG. 4 shows a procedure for receiving the connect message according to the prior art. This operation is performed when the BSC-SDU receives the connect message for requesting channel assignment, transmitted from the BTS, and generates a connect ACK message for the connect message.
Referring to FIG. 4, the BSC-SDU receives the connect message for requesting channel assignment from the BTS in step 401. In step 401, the BSC-SDU analyzes the received connect message, and examines the establishment-requested channel in the message of FIGS. 5A and 5B and an identifier of a traffic channel between the BTS and BSC. The BSC-SDU assigns the traffic channel between the BSC and BTS, corresponding to the radio channel, in step 403. As a result, channel connection among BSC-BTS-MS is completed. Further, in step 405, the BTS-SDU fills all the information elements of the connect ACK message shown in FIG. 6 and transmits it to the BTS.
A simplified structure of the connect ACK message shown in FIG. 6 will be described with reference to Table 1 below.
The connect message shown in Table 1 is an A3 message transmitted when the target BS 40 initiates or adds one or more A3 user traffic connections to the SDU 34 of the source BS 30. The A3 message includes the following information.
Message Type II: an information element indicating A3/A7 message type.
Call Connection Reference: an information element for uniquely defining a call connection over all zones. This value is always maintained during call connection over every handoff.
Correlation ID: an information element used to correlate a request message with a response message for the request message.
SDU ID: an information element for identifying a specific SDU instance in one SDU node.
A3 Connect Information: an information element used to add one or more cells to one new A3 connection or existing A3 connection. This information element field is shown in Table 2 below, and 4th to (jxe2x88x921)th octets of Table 2 include Cell Information Record fields of Table 3 below.
Table 3 shows a message including air interface channel information for the cells attached to one call leg, and each field is defined as follows.
Length: the number of octets of the elements following a Length field.
Cell Identification Discriminator: a value used to describe the formats following a Cell Identification field according to cells.
Cell Identification: identification of the cells relating to A3 connection.
Reserved: this value is set to xe2x80x9800000xe2x80x99.
New Cell Indicator: a field indicating whether a corresponding cell is a cell newly added to A3 traffic connection in the present procedure or a cell which previously exists in A3 connection.
PWR_Comb_Ind: a power control symbol combining indicator. The BTS sets this field to xe2x80x981xe2x80x99, if a forward traffic channel relating to the corresponding pilot transmits the same bits as closed-loop power control subchannel bits of a previous pilot in this message. Otherwise, the BTS sets this field to xe2x80x980xe2x80x99. When this record occurs first in this element, the BTS sets this field to xe2x80x980xe2x80x99.
Pilot_PN: this field includes a PN sequence offset corresponding to the related cell and is set in a unit of 64 PN chips.
Code_Chan: this field includes a code channel index corresponding to the related cell. The BTS sets a value used on the forward traffic channel in connection with a designated pilot to one of 0 to 63.
A simplified structure of the connect ACK message shown in FIG. 6 will be described with reference to Table 4 below.
The connect ACK message of Table 4 is an A3 message for transmitting A3-CDMA Long Code Transition Directive results performed on the A3 signaling interface from the target BS 40 to the SDU 34 of the source BS 30. Further, an A3 CDMA Long Code Transition Directive Ack message for the A3 CDMA Long Code Transition Directive of Table 4 is shown in Table 5 below.
The A3 CDMA Long Code Transition Directive Ack message of Table 5 includes the following information elements.
Message Type II: an information element indicating an A3/A7 message type.
Call Connection Reference : an information element for uniquely defining a call connection over all zones. This value is always maintained during call connection over every handoff.
SDU ID: an information element for identifying a specific SDU instance in one SDU node.
PMC Cause: an information element indicating failed results of A3/A7 message.
Cell Information Record (Committed, Uncommitted): a Cell Information Record field of Table 3 is used, as it is. This is an information element including air interface channel information for the cells attached to one call leg. When successful, this field is set to xe2x80x98Committedxe2x80x99, and when failed, this field is set to xe2x80x98Uncommittedxe2x80x99. This field is used together with the PMC Cause field.
Problems of the existing channel assignment method will be described based on the foregoing descriptions.
As described with reference to Tables 1 to 5, in the conventional 3G IOS radio channel information, the quasi-orthogonal function (QOF) specified in the CDMA-2000 standard is not defined. Further, only 64 Walsh codes of 0 to 63 are supported for the code channels. Thus, when the base station does not support the quasi-orthogonal function (QOF) which is necessary for the mobile station, the 3G forward radio channel cannot be assigned in the mobile station. Therefore, it is necessary to define a message field which can support the quasi-orthogonal function (QOF) and 256 Walsh codes for the radio channel information in the existing 3G IOS.
It is, therefore, an object of the present invention to provide a method for expanding existing 64 supportable Walsh codes to 256 Walsh codes and supporting a quasi-orthogonal function (QOF) for a forward channel in a base station of a mobile communication system.
To achieve the above objects, there is provided a method for performing channel assignment in a base station for a mobile communication system. Upon receipt of a request for assigning a channel to a mobile station, a base station transceiver system (BTS) generates a connect message including channel information indicating a Walsh code to be used for a channel to be assigned to the mobile station, out of 256 Walsh codes, and information indicating a quasi-orthogonal function (QOF) index, and transmits the generated connect message to a base station controller (BSC). The BTS generates a connect ACK message for acknowledging the channel assignment-related information included in the connect message and transmits the generated connect ACK message to the BSC. Upon receipt of the connect ACK message, in the BTS assigns a channel acknowledged by the BSC to the mobile station.